fix(saas): eliminate 6 compiler warnings + stabilize directive complete

- Remove unused imports: Utc (billing/service), StatusCode (billing/handlers), Sha256 (billing/handlers)
- Fix unused variables: _db (scheduler), _e (payment WeChat error)
- Fix visibility: RegisterDeviceRequest pub(super) → pub (used in pub handler)
- Update STABILIZATION_DIRECTIVE.md: all 7 criteria met, downgrade to advisory
- Fix TRUTH.md §2.2: mark P0/P1 defects as resolved, update Admin pages count to 14

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/zclaw-saas/src/billing/handlers.rs

@@ -2,11 +2,9 @@
 
 use axum::{
     extract::{Extension, Form, Path, Query, State},
-    http::StatusCode,
     Json,
 };
 use serde::Deserialize;
-use sha2::Sha256;
 
 use crate::auth::types::AuthContext;
 use crate::error::{SaasError, SaasResult};